Virginia regulators on Thursday refused to let Walmart leave its incumbent utility service in the state to shop for electricity on the open market, writing it would raise costs too much for residential ratepayers who cannot choose their power supplier.Walmart’s exit from the services of Dominion Energy and Appalachian Power would cost residential ratepayers nearly $70 million over 10 years, the State Corporation Commission (SCC) said. That “cost shifting” is not “consistent with the public interest,” they wrote.